Obligate costs of parental care to offspring: Egg brooding-induced hypoxia creates smaller, slower and weaker python offspring

Python egg brooding typifies parental care because it consists of multiple behaviours that provide for multiple developmental needs. For example, tightly coiling around the eggs benefits embryonic water balance, but periodic female postural adjustments improve embryonic gas exchange. Regardless of these postural adjustments, egg brooding creates a hypoxic intra-clutch environment that constrains embryonic metabolism. We further examined this novel and useful parental care model to determine: (1) any fitness-related costs of egg brooding to offspring; (2) whether any long-term costs are alleviated by postural adjustments. We artificially incubated Children's python (Antaresia childreni) clutches and modulated oxygen partial pressure (PO₂) to create three treatments: normoxic (NRM, 20.3 kPa O₂), brooding [BRD, PO ₂ profile typical of clutch PO₂ (PO_2clutch) in maternally brooded clutches, 15.8-19.3 kPa O₂] and low (LOW, predicted PO₂ profile of maternally brooded PO_2clutch if females did not make postural adjustments, 14.4-18.6 kPa O₂). Using various metrics from ∼12 days pre-hatching to 14 days post-hatching, we demonstrated that NRM offspring were larger, faster and stronger than BRD offspring. As only hatchling heart mass differed between BRD and LOW treatments (LOW > BRD), postural adjustments may not alleviate hypoxia-related costs to embryos. Our results demonstrate that parental care may represent a compromise between competing developmental needs and thus entails obligate costs to the offspring.